1. Field of Invention
The present invention relates to an electric detonating fuse assembly, and more particularly to a new and improved electric detonating fuse assembly capable of preventing abnormal ignition such as an accidental discharge caused from static electricity.
2. Prior Art
Generally, the electric detonating fuse assembly or squib has been widely used in the past as an igniting device which causes to explode an explosive compound, a rocket propellant and the like. Particularly, in recent years it serves as one which in turn ignites a trigger actuator of a protective air bag or expansible confinement for protecting an occupant or occupants of a vehicle from severe vibration and shock conditions upon the occurrence of a collision. As best seen in such applications, it is required that the electric detonating fuse assembly provide extremely high safety and reliability of operation. For example, it will be apparent that when the trigger actuator of a protective air bag is suddenly ignited, irrespective of the sensing of any shock conditions of the vehicle, to expand the protective air bag, very dangerous conditions may be encountered with respect to the occupants of the vehicle. Therefore, it is required that the electric detonating fuse assembly provide a safe construction and electric circuitry of the type which prevents such abnormal ignition thereof. In general, the electric detonating fuse assembly comprises a construction in which an igniting charge enclosed in the tubular body member of the fuse assembly is to be ignited upon application of electric current through a filament positioned in the vicinity of the igniting charge. It has been well known that in the electric detonating fuse assembly of this type that the abnormal ignition thereof is mainly caused by the action of the static electricity being accumulated. When a static voltage charged in a filament electrode or a metallic tubular body member reaches a certain level of voltage, a discharge occurs between them. If this discharge energy exceeds a predetermined value, the igniting charge is activated to give rise to an abnormal ignition. Particularly, the trigger actuator for the protective air bag is susceptible to be charged with static electricity since it is usually connected in the vehicle where static electricity is very likely to be produced and stored cumulatively. Under these use conditions, serious problems are encountered due to such abnormal ignition caused by the accumulated static electricity.
It is impossible to prevent the charging action itself described above. However, if the discharge energy is reduced, abnormal ignition will not be caused. Based on this, the prior art electric detonating fuse assembly is so constructed that spark gap is provided between the filament electrode and the tubular body and with sufficient separation from the igniting charge. This arrangement ensures that before reaching a critical level of potential to cause an abnormal ignition, the accumulated static electricity is discharged through a spark discharge occurring earlier at the spark gap. Such conventional apparatus normally needs to provide a spark gap of the type having a low firing potential. It has been, however, very difficult to fabricate such a discharge gap and yet have high accuracy in construction. Particularly since the firing potential of the discharge gap is sensitive to the surface conditions of the discharge electrodes and their gap distance, it has been difficult to consistently maintain the discharge voltage thereof at a certain fixed value. In the event, also, that electric charges are concentrated on the discharging surface in a short time, it is feared that high voltage discharge will be effected through the spark gap between the filament electrode and the tubular body member of the electric detonating fuse assembly thereby causing a discharging delay. Therefore, it was impossible to prevent the abnormal ignition completely.